PHREEQC Pitzer database

Quintessa has reviewed the completeness of the thermodynamic database developed by the Yucca Mountain project and developed software to convert the database format.

In 2008, Quintessa was commissioned by NWMO to convert the database format from its native EQ3/6 format into PHREEQC format. The associated work is documented in a Quintessa technical memorandum. The PHREEQC formatted version of the database can be obtained by contacting Steven Benbow. As noted in the Quintessa technical memorandum, there are known limitations with the database and any application must be undertaken with care to ensure that the database is used in an appropriate manner.

The Yucca Mountain database includes “Pitzer data” to parameterise the activity coefficients of aqueous species. Traditional approaches to defining activity coefficients, such as the Debye-Huckel or Davies approaches, are only applicable to dilute aqueous solutions with ionic strengths up to about 0.8. For solutions of higher salinity, the “Pitzer approach” to defining activities based on specific ion-interactions is sometimes more appropriate and can provide more accurate modelling results.

The Yucca Mountain database is stored in EQ3/6 format. To enable NWMO to use the Yucca Mountain project database in their preferred geochemical modelling code, PHREEQC, Quintessa developed software to automatically convert the database from its native EQ3/6 format into PHREEQC format. The software was developed under Quintessa’s ISO 9001:2000 approved software procedures that meet the TickIT software development standards.